1. Field of the Invention
This invention relates to index of refraction sensors and, more particularly, to a critical angle refractometer in which the wavelength rather than the angle for critical reflection of the incident energy is continuously measured as a function of the external index of refraction.
2. The Prior Art
In conventional refractometers the variation of the critical angle for monochromatic incident radiant energy is measured as a function of the external index of refraction. Examples of these Abbe laboratory refractometers are Gaertner Scientific corporation's model No. L128-94 and American Optical's model No. 10450. An example of an in-situ Abbe refractometer utilizing an optical fiber to direct the monochromatic radiant energy to the sensor and an electrical wire to direct the voltage signal from the sensor to the detector is described in Mahrt, et. al. (1982).
Other examples of the principle of the Abbe refractometer are found in Hansen (1965) and Tako et al. (1975). These last two examples also incorporate some aspects of the critical wavelength technique as well, although their accuracy and resolution is quite limited and they do not measure continuously.
Other refractometers use broadband, non-coherent, non-collimated radiant energy and measure the intensity of the transmitted or reflected radiant energy as a function of the external index of refraction of the fluid; these refractometers can read remotely from the radiant energy source and sensor, usually through optical fiber transmission links, but have drift and calibration difficulties. Examples of this approach are found in Abuaf, et. al. (1978) in which incandescent radiant energy is guided through multimode optical fibers to a 45.degree.-90.degree.-45.degree. glass prism and returned to a detector through a second multimode fiber. The radiant energy is reflected from the prism when it is in air and refracted out when it sees water; it is used to detect water in two phase flows. Another example is that of Harmer (1983) in which radiant energy from a light emitting diode is guided through a multimode optical fiber which is bent into an "S" shape in the region in which it is desird to measure the index of refraction. The radiant energy that is not refracted out in the "S" portion is returned to the detector through a straight multimode optical fiber. The detector then relates the intensity of the incident radiation to the index of refraction external to the "S" region.
Arrington (1981) and Uramoto (1977) are also examples of the above intensity-modulated refractometers in which critical reflection plays a minor role.
To be completely useful and widely accepted in such fields as oceanography, it is desirable that the refraction sensor be less than 0.6 mm in diameter, have a stable calibration with an accuracy in the index of refraction of 1.times.10.sup.-5, be capable of reading remotely, and continuously, be inexpensive and simple with the possibility for expendable use, and be suitable for use in the oceans. Such an index of refraction sensor is disclosed and claimed herein.